Some engineering features of the Weymouth station of the Edison Electric Illuminating Co. of Boston

The new power station of The Edison Electric Illuminating Company of Boston, now under construction on the Weymouth Fore River is expected to have an ultimate capacity of about 300,000 kw. The present construction covers the installation of two 32,000 kw. generating units with three 1974-h. p. boilers, operating at 375 lb. steam pressure and 700 deg. fahr. In addition, a single boiler to carry steam at pressures up to 1200 lb. is to be installed. This boiler will have about the same heating surface as the normal pressure boilers. The steam generated by it will pass through a pressure reducing turbine developing about 2000 kw., and will be exhausted at 375 lb. pressure. After being reheated to the original temperature of 700 deg. the steam will be piped to the main header and used in the large turbines. If satisfactory results are obtained from the higher-pressure boiler-turbine units more of them will be installed. Three will be required to furnish sufficient steam to operate one of the 32,000-kw. units. The maximum steam temperature was fixed at 700 deg. in consideration of the properties of materials at the higher temperatures. While the theoretical gain from higher pressure increases up to the maximum for which any data are available, the full benefit of the thermodynamic possibilities are not at present obtainable in practise without reheating the steam at some point intermediate between the throttle and the condenser. Without reheating, the most advantageous steam pressure, both practically and economically considered, seems to be about 375 lb. gage. With reheating, it appears that about 1200 lb. is a practicable initial pressure, and that 375 lb. is an entirely satisfactory pressure at which to reheat. Accordingly a combination of these two pressures with intermediate reheating has been adopted; the higher pressure with a view to developing its possibilities, the lower pressure with the feeling that it represents the best practise in the single expansion cycle and that operation solely at that pressure will give highly economical results. The character of the future development of the plant will depend upon the relative performance of the two classes of equipment. Feed water is to be heated by two-stage bleeding of the main units and by economizers. All normally running auxiliary equipment is to be driven by alternating-current motors. Power for driving the essential auxiliaries is to be supplied from a 2300-volt alternator of 2500 kv-a. capacity connected directly to each 30,000-kw. main generator shaft, and thus driven by the main turbine, which will have sufficient capacity for driving both generators at full load. This arrangement is expected to give a combination of greater economy and security than any heretofore used. The switch house will be a separate structure, four stories and basement. The upper floor will carry the switchboard and the switch operating mechanism. The three lower stories will house the bus structures, oil circuit breakers and reactors, each phase being isolated to a single floor. The basement will serve as a cable vault and all incoming and outgoing circuits will pass through it. There will be two main ring busses and a transfer bus. The ring busses will be divided into sections, each section fed by two generators and with current-limiting reactors between sections. All circuits will be connected to the busses through two oil circuit breakers in series. No prediction is made of the expected operating performance of the station, but it is calculated that under ideal conditions, acting solely as a 375-lb. single-expansion nonreheating plant, it could produce a net kw-hr. for 15,100 B. t. u. in the fuel and as a 1200 lb. compound reheating plant, it could produce a net kw-hr. for 13,600 B. t. u. These figures show the advance in heat economy that modern developments in steam engineering have made possible and the advantage which the very high-pressure reheat cycle possesses over the more conventional design. The actual plant performance will depend among other things upon the character of the load which it is found possible to put on Weymouth Station and the relative proportion of high and normal pressure equipment that may be operated.